1. Field of the Invention
The present invention relates to a cantilever for a scanning probe microscope, and more particularly, to a structure of and a manufacturing method for the cantilever of this type which comprises a lever section and a probe section disposed near the free end thereof.
2. Description of the Related Art
A scanning tunneling microscope (STM), which had been invented by G. Binning, H. Rohrer, Ch. Gerber, and E. Weibel (Surface Studies by Scanning Tunneling Microscope, Phys. Rev. Lett., 49 (1982) 57), has been used in a wide variety of fields, serving as a microscope through which atomic-order indentations of a surface can be observed.
Meanwhile, an atomic force microscope (AFM) was proposed as a further developed version of the STM (Published Unexamined Japanese Patent Application No. 62-130302; Method and Apparatus for Forming Image of Sample Surface by G. Binning, IBM). With use of the AFM, an insulating sample, which has not been able to be measured by means of the STM, can be observed with an accuracy of atomic orders, utilizing elemental technology including servo technology. FIG. 15 shows an example of the AFM disclosed in the patent application mentioned above. The AFM resembles the STM in structure.
In FIG. 15, a cantilever 11 having a sharp projection (first probe section 13) at its tip end is disposed opposite and close to a sample 9. As the distal end of the probe section 13 brought close to the sample, its atoms and the atoms of the sample interact with one another, thus producing a force proportional to the distance between the section 13 and the sample. If the sample and the probe section are relatively scanned in the XY direction in this state, a lever section 12 shifts its position depending on the irregularity of the sample surface. Thus, an insulating sample, which has not been able to be measured by means of the STM, can be measured indirectly by determining the displacement of the lever section with use of an AFM system which includes a second probe section 14 located on the opposite side of the cantilever 11 to the sample 9.
In an FM (force microscope) typified by the AFM described above, the shape of the probe section, for use as its sensor element, is a very important factor, since it may greatly influence resolutions in the X, Y, and Z directions or restrict the types of measurable samples.
Conventional cantilevers are manufactured by sharpening an end of a thin metal wire by mechanical or electropolishing, and if necessary, bending the wire into the shape of an L, or by bonding a diamond piece, for use as a probe section, to a leaf of metal or the like.
According to the FM with one such cantilever, however, it is difficult to manufacture the lever section of the cantilever with high accuracy, and the measurement conditions inevitably vary when the cantilever, an expendable,-is replaced with a new one. The mechanical resonance frequency of the cantilever depends on its length, so that the replacing cantilever should have the same resonance characteristic and the same high-accuracy length as the replaced one. According to the method described above, however, such high-accuracy cantilevers cannot be obtained with ease. For higher resistance to external vibration, moreover, a shorter lever section is preferred because it can ensure a higher resonance frequency. According to the aforementioned method, however, it is also difficult to form a short lever section.
T. R. Albrecht et al. reported a novel cantilever which has a pyramid-shaped probe section (Thomas R. Albrecht, Shinya Akamine, Marco Tortonese, and Calvin F. Quate; Advances in Atomic Force Microscopy, STM '89 Poster Session). This pyramid-shaped probe section is manufactured by subjecting a lattice plane having a Miller index (100) of a silicon wafer to wet anisotropic etching to form a pyramid-shaped hole, forming an Si.sub.3 N.sub.4 film on the resulting structure, and then etching the structure into the form of a lever. This microcantilever, which is manufactured using the same processes for the manufacture of a semiconductor IC, can enjoy high accuracy of .mu.m orders and very high reproducibility. However, even this method cannot always provide a satisfactory probe section.
The point angle of the probe section restricts the types of measurable samples. Accordingly, a sample 16 having a hole or groove 17 which is so deep that the tip end of a probe section 15 cannot reach the bottom of the hole 17, as shown in FIG. 12, cannot produce a reliable output signal, as shown in FIG. 13, so that its measurement is impossible. Also, a steep stepped structure will become as dull as the probe angle. As shown in FIG. 14, therefore, the point angle of the probe section 18 should be as narrow as possible. Since a pyramid-shaped probe section is formed as a replica of an etching pit in silicon, however, its individual faces cross at angles of 72.degree. to one another, corresponding to the crystal structure of silicon. The maximum possible angle formed between any two of four edges which extend As compared with a point angle of 30.degree. or less for an electropolished probe used in the STM, the point angle of the pyramid-shaped probe section is so wide that it is difficult to measure samples having deep holes or grooves. Thus, the shape of a pyramid has never been regarded as a desired shape for the probe section.
Accordingly, the cantilever has been expected to be manufactured with high accuracy by utilizing the semiconductor process as aforesaid, and to have a probe section with a narrow point angle.
The point angle mentioned herein is the angle 8 of the distal end portion of a probe section 19, and is different from the radius R, as shown in FIG. 16.
Further, T. R. Albrecht et al. reported a cantilever which has a probe section with a narrow point angle (Thomas R. Albrecht and Calvin F. Quate; Atomic resolution with the atomic force microscope on conductors and nonconductors, J. Vac. Sci. Technol. A6(2), (1988) 271). In this case, however, the probe section is not long enough to be a satisfactory one. Thus, there is a demand for a cantilever having a longer probe.
If the probe section of the cantilever is not long enough, the cantilever must be inclined with respect to the surface of a sample when it is mounted on the FM to be used for measurement, in order to prevent the sample from touching the other portion of the cantilever than the probe section. Therefore, the design and manufacture of the FM are difficult or troublesome. This is a critical problem, in particular, for an FM which is designed to measure the displacement of the cantilever optically. If the cantilever is inclined, moreover, the motion of the probe section involves the motions of components in other directions than the one perpendicular to the sample surface. Therefore, the irregularity of the sample surface cannot always be ideally detected, so that the resolution is low. Thus, there is a demand for the development of a longer probe section with a sharp tip configuration, as well as for solutions to the aforementioned problems.
Since cantilevers are regarded as expendables, furthermore, they are expected to be manufactured with reliability and at low cost.